fork(), read(), write(), O_APPEND 使用
互应Lab03 fork(), read(), write(), O_APPEND 练习
运行示例程序
fork1.c
linux中fork（）函数详解 - 学习记录园 - 博客园
fork调用仅被调用一次，能返回两次，可有三种返回值：
1）在父进程中，fork返回新创建子进程的进程ID；
2）在子进程中，fork返回0；
3）如果出现错误，fork返回一个负值；
创建新进程成功后，系统中出现两个基本完全相同的进程，这两个进程执行没有固定的先后顺序，哪个进程先执行要看系统的进程调度策略。
两个进程的变量都是独立的，存在不同的地址中，不是共用的，这点要注意。
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main(void){
pid_t t;
// fork is going to create a child process
t=fork();
printf("fork returned %d\n",t);
// exit is to terminate a process and release all the resources
exit(0);
}
fork2.c
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main(void){
pid_t t;
printf("Original proram, pid = %d\n",getpid());
t = fork();
if(t==0){
printf("In child process, pid = %d, ppid=%d\n",getpid(),getppid());
}
else{
printf("In parent, pid = %d, fork returned = %d\n",
getpid(),t);
}
}
结果：
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ ./fork2.out
Original proram, pid = 3921
In parent, pid = 3921, ppid=3893, fork returned = 3922
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ In child process, pid = 3922, ppid=1110
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ gcc fork2.c -o fork2.out
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ ./fork2.out
Original proram, pid = 4041
In parent, pid = 4041, fork returned = 4042
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ In child process, pid = 4042, ppid=1110
发现：
在虚拟机上创建新的进程和理论上是不一样的。这个问题需要探讨，可能是因为虚拟机请求主系统创建，那个pid是主系统的？
感觉Ubuntu打开终端老是要搞路径很麻烦，下载nautilus-open-terminal。
ubuntu14设置在当前目录打开终端_xbcReal的博客-CSDN博客
exec1.c
#include<unistd.h>
#include<stdio.h>
int main (void){
char *arg[] = {"/bin/ls",0};
/* fork, and exec within child process*/
if(fork()==0){
printf("In child process:\n");
execv(arg[0],arg);
printf("I will never be called\n");
}
printf("Execution continues in parent process\n");
}
输出结果：
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ ./exec1.out
Execution continues in parent process
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ In child process:
exec1.c exec1.out fork1.out fork2.c~
exec1.c~ fork1.c fork2.c fork2.out
lseek1.c
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h> //exit()
char buf1[] ="abcdefghij";
char buf2[]="ABCDEFGHIJ";
#define FILE_MODE 0644
int main (void){
int fd;
if((fd = creat("file.hole",FILE_MODE))<0)
{
printf("creat error\n");
exit(1);
}
if(write(fd,buf1,10)!=10){
printf("buf1 write error\n");
exit(1);
}
/*offset now = 10*/
if(lseek(fd,40,SEEK_SET)==-1){
printf("lseek error\n");
exit(1);
}
/*offset now = 40*/
if(write(fd,buf2,10)!=10){
printf("buf2 write error\n");
exit(1)；
}
/*offset now = 50*/
exit(0);
}
输出结果：
kunling@kunling-VirtualBox:~/Internet_Application/lab03$ od -c file.hole
0000000 a b c d e f g h i j \0 \0 \0 \0 \0 \0
0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
0000040 \0 \0 \0 \0 \0 \0 \0 \0 A B C D E F G H
0000060 I J
0000062
readwrite1.c
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
int main(void){
char quit='.';
char buf[10];
int fd;
if((fd=open("out.out", O_RDWR| O_CREAT))==-1)
printf("Error in opening\n");
while(buf[0]!= quit) //遇到‘.'就结束!
{
read(0 ,buf, 1);
write(fd,buf, 1);
write(1 ,buf, 1);
}
close(fd);
}
输出结果：
root@kunling-VirtualBox:/home/kunling/Internet_Application/lab03# ./readwrite1.out
1234567890
1234567890
qwertyuio
qwertyuio
ff.
ff.root@kunling-VirtualBox:/home/kunling/Internet_Application/lab03#
root@kunling-VirtualBox:/home/kunling/Internet_Application/lab03# od -c out.out 0000000 1 2 3 4 5 6 7 8 9 0 \n q w e r t
0000020 y u i o \n f f .
0000030
验收O_APPEND的特性
lseek的与Ｏ_APPEND的冲突_大海蓝天的专栏-CSDN博客
-
读取特性
按照lseek文件指针读取
#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<string.h>
int main(){
char rbuf1[100];
int fd;
int size;
int currpos;
//在当前目录创建或者打开文件
fd = open("testing.txt",O_RDWR| O_APPEND);
if(fd<0)
printf("open failed");
//移动文件指针到开头
currpos = lseek(fd, 0 , SEEK_SET);
if(currpos!=0){
printf("lseek error");
return -1;
}
if(read(fd,rbuf1,sizeof(rbuf1))<0)
printf("read error!");
//打印读取到的文件内容
printf("%s",rbuf1);
close(fd);
return 0;
}
-
写入特性
lseek文件指针失效，在文件末尾附加。
#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<string.h>
#include <stdlib.h>
int main(){
char str_write1[] = "999";
char str_write2[] = "000";
char rbuf1[100];
char rbuf2[100];
int fd;
int size;
int currpos;
//在当前目录创建或者打开文件
fd = open("testing.txt",O_RDWR|O_APPEND|O_CREAT, 0644);
if(fd<0){
printf("open and creat failed");
}
//移动文件指针到开头
currpos = lseek(fd, 0 , SEEK_SET);
if(currpos!=0){
printf("lseek error");
return -1;
}
//打印当前文件指针位置
printf("file pointer position: %d\n",currpos);
//向文件中写入
size = write(fd, "999", strlen("999"));
close(fd);
return 0;
}
动文件指针到开头
currpos = lseek(fd, 0 , SEEK_SET);
if(currpos!=0){
printf(“lseek error”);
return -1;
}
//打印当前文件指针位置
printf("file pointer position: %d\n",currpos);
//向文件中写入
size = write(fd, "999", strlen("999"));
close(fd);
return 0;
}
